Wireless LAN data rate adaptation

ABSTRACT

A WLAN communication system and algorithm that adaptively changes the data transmission rate of a communication channel based on changing channel conditions. The WLAN communication system or algorithm has two modes being a searching mode and a transmission mode. Furthermore, the WLAN communication system or algorithm incorporates an additive increase, multiplicative decrease (AIMD) function into the rate adaptation algorithm.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to adapting or adjusting a data rate tomaximize overall performance of a wireless local-area-network (WLAN). Inparticular, the present invention relates to a method for adjusting thedata communication rate of an IEEE 802.11 standard WLAN connection orsystem.

2. Description of Related Art

The IEEE 802.11 standard for data communication provides a multi-ratecapability. Transmission of data can occur at any of the IEEE 802.11standard's specified allowable data rates. In a wireless environment,communication and channel conditions effect the maximum usabletransmission rate. The multi-rate capability of the IEEE 802.11 standardis supported at the physical layer. In particular, Media Access Control(MAC) mechanisms are used to exploit the multi-rate capability andmaximize the overall data and information throughput of a WLAN. The IEEE802.11 standard does not specify how to exploit a maximum data rate fora communication channel's condition. The IEEE 802.11 standard leaves itto the individual developers to devise rate adaptation techniques thattake advantage of the standard's multi-rate data communicationcapability.

To date, various rate adaptation schemes have been proposed and usedwith the IEEE 802.11 standard. The existing rate adaptation schemes canbe classified into two categories, the first category being schemes thatuse the success and failure history of previous transmissions to aid inthe selection of future data or communication rates. This first categorymay also be known as Auto Rate Feedback (ARF). The second category ofrate adaptation schemes utilize signal measurements to aid selection ofan appropriate data rate.

Providing more background about existing, first category, ARF schemes,the article Ad Kamerman et al, “Wave LAN-II: A High-Performance WirelessLAN for the Unlicensed Band,” Bell Labs Technical Journal, pp. 118-133,Summer 1997 discusses a proposal for Lucent's WaveLAN-II devices andother commercial WLAN products. The described ARF style WLAN adaptationdevice basically switches between 1 and 2 Mbps transmission rates basedon a timing function and the number of ACK frames that are not receivedback by the sending device. The default transmission rate is the higherrate. The transmission rate switches to the lower rate when twoconsecutive ACKs are not correctly received by the sending device. Atthe same time that the transmission rate switches to the lower rate, atiming function begins. The transmission rate will switch back to thehigher transmission rate after a predetermined amount of time or whenthe number of consecutively, correctly received ACKs is equal to ten.This ARF scheme is easy to program and implement without modification tothe current IEEE 802.11 standard. The disadvantage of this ARF scheme isthat it cannot and does not react quickly to fluctuations in channelconditions.

Another ARF style rate adaptation scheme is proposed in P. Chevillat etal, “A Dynamic Link Adaptation Algorithm for IEEE 802.11a WirelessLANs,” IEEE International Conference on Communications (ICC), pp.1141-1146, 2003. This ARF scheme uses information, like ACKs, onlyavailable to the information sender, to estimate the channel condition.Thus, there is no feedback channel from the receiving device. Thisscheme uses only one failure threshold F and two success thresholds S,and S₂, correspond to different channel conditions: a channel conditionin a region of a higher Doppler Spread value (S₁=3) or in a region oflower Doppler Spread value (S₂=10).

Yet another discussion of ARF style rate adaptation schemes is discussedand evaluated in A. J. van der Vegt, “Auto Rate Fallback Algorithm forthe ARF style rate adaptation scheme is very similar to the LucentWaveLAN-II, discussed above, except for there being a different choiceof fixed threshold values for determining when to switch thetransmission rates to a higher or lower rate.

We now provide additional background on the second category of rateadaptation schemes, the schemes that utilize signal measurements to aidselection on an appropriate transmission data rate. In this category ofrate adaptation schemes, the algorithms assume that there is additionalcommunication between the sender and receiver regarding thecommunication link or channel condition. As a result, this type of rateadaptation scheme requires modifications or additions to the IEEE802.11a standard. For example, the Receiver-Based Auto-Rate (RBAR)protocol, as discussed in Gavin Holland et al, “A Rate-Adaptive MACProtocol for Multi-Hop Wireless Networks,” ACM MobiCom '01, pp. 236-251,2001, requires using an RTS/CTS handshaking process, which is not partof the IEEE 802.11a standard. In this example, RBAR the receiverestimates the wireless channel condition by using a sample of theinstantaneously-received signal strength from the end of the RTSreception. The RBAR protocol then feeds the transmission rate back tothe sending device using the CTS packet portion of the RTS/CTShandshaking process.

Yet, another signal measuring-based rate adaptation scheme is referredto as the Opportunistic Adaptive Rate (OAR) scheme which transmitsmultiple data packets when the scheme believes that the channelcondition is good and thereby achieves a slightly better throughput thanthe RBAR scheme.

In the Daji Qiao, et al, “Goodput Analysis and Link Adaptation of IEEE802.11a Wireless LANs,” IEEE Trans. On Mobile Computing, vol. 1, no. 4,pp. 278-292, October-December 2002, article a ‘goodput’ analysis forrate adaptation in an 802.11a WLAN is presented. This article suggests acoupling of the transmission rate and the data fragment size to thechannel condition estimation. A mathematic model is used with the datafragment size to aid in computing the best rate. A drawback of this rateadaptation scheme is that it assumes perfect channel knowledge and thusis of little practical use.

Another proposed rate adaptation algorithm by Javier de Prado et al,“Link Adaptation Strategy for IEEE 802.11 WLAN via Received SignalStrength Measurement,” IEEE International Conference on Communications,May 2003, proposes to use the Received Signal Strength (RSS) of receiveddata frames, along with the number of retransmissions, to estimate thechannel condition. This algorithm does not require any coordination fromthe receiver and thus does not require a change to the IEEE 802.11standard. However, this scheme assumes that the signal strength of thesending device is the same as the signal strength at the receiver. Inother words, the scheme assumes that a symmetrical channel conditionexists. As a result that is not a practical scheme.

What is needed is a rate adaptation scheme that readily adapts to thechannel conditions of a WLAN system. Such a rate adaptation schemeshould be relatively easy to implement without requiring changes ormodifications to the IEEE 802.11 standard.

BRIEF SUMMARY OF THE INVENTION

Embodiments of the invention provide a flexible rate adaptationalgorithm that adjusts the data transmission rate mode based on channelconditions of a channel in a WLAN system. An embodiment of the presentrate adaptation scheme further adjusts the calculation for determiningan appropriate data transmission rate mode as channel conditions change.Furthermore, embodiments of the invention are basically an ARF stylealgorithm and scheme.

An embodiment of the invention provides a WLAN system that has a sendingstation and a receiving station. The sending station is programmed tosend WLAN communications using an adaptive rate algorithm. The adaptiverate algorithm has a searching mode for searching for an appropriatedata transmission rate mode, which provides the fastest, successful WLANcommunication between the sending station and the receiving station. Theadaptive rate algorithm also has a transmission mode wherein the datatransmission rate mode remains constant until a predetermined number ofunsuccessful transmission attempts occur. Furthermore, in thetransmission mode, the transmission rate also remains constant until apredetermined number of successful transmissions occur therebyencouraging the exemplary transmission scheme to increase the datatransmission rate to a higher data transmission rate mode.

Other embodiments of the invention provide a method of communicating ina WLAN system. The method of communicating includes a sending devicethat sends a data frame to a receiving device at a current datatransmission rate mode (data rate). The sending device determineswhether an acknowledge signal (ACK) is received at the sending device.If an ACK is received by the sending device, then a success thresholdcounter is incremented. If the success threshold counter becomes equalto a success threshold number (TH), then the current data transmissionrate mode is increased to a new data transmission rate mode equal to((the current data transmission rate mode number)+((the number ofpossible data transmission rate modes)−(the current data transmissionrate mode number))÷2). The success threshold counter is also reset tozero. Conversely, if the success threshold counter is not equal to thesuccess threshold number, then the success threshold counter isincremented by one and the next data frame is sent from the sendingdevice to the receiving device.

Yet other embodiments of the present data rate adaptation system providea WLAN system that includes a sending device for sending wirelesscommunications in a WLAN network on a channel. The sending device isconfigured or programmed to communicate in the WLAN system using a twophase communication algorithm. The two phases are a searching phase anda transmission phase such that an additive increase—multiplicativedecrease (AIMD) algorithm is employed.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosed invention will be described with reference to theaccompanying drawings, which show important sample embodiments of theinvention and which are incorporated in the specification hereof byreference, wherein:

FIG. 1 is a is a WLAN configuration;

FIG. 2 is a flow chart of a transmission success scheme according to anembodiment of the invention;

FIG. 3 is a flow chart of a transmission failure scheme according to anembodiment of the invention;

FIG. 4 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Flat Fading with a Doppler spread of 1 Hz are present;

FIG. 5 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Flat Fading with a Doppler spread of 5 Hz are present;

FIG. 6 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Flat Fading with a Doppler spread of 10 Hz are present

FIG. 7 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Flat Fading with a Doppler spread of 20 Hz are present;

FIG. 8 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Flat Fading with a Doppler spread of 1, 5, 10 and/or 20 Hz;

FIG. 9 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Dispersive Fading with a Doppler spread of 1 Hz are present;

FIG. 10 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Dispersive Fading with a Doppler spread of 5 Hz are present; and

FIG. 11 is a chart showing the data throughput improvement (%) of anembodiment of the invention compared to prior algorithms when conditionsof Dispersive Fading with a Doppler spread of 10 Hz are present.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS

The present invention will now be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

WLAN topology is fairly straight forward. WLANs allow workstations (orany device with wireless capability) to communicate and to access anetwork using radio propagation as the transmission medium. A WLAN canbe connected to an existing wired LAN as an extension, or can form thebasis of a new network. While adaptable to both indoor and outdoorenvironments, WLANs are especially suited to indoor locations such asoffice buildings, manufacturing floors, homes, hospitals, anduniversities.

The basic building block of a WLAN is a “cell”. The cell is the area inwhich wireless communication takes place. The coverage area of a celldepends on the strength of the propagated radio signal and the type ofthe construction of the walls, partitions, and other physicalcharacteristics and electronic noise of the environment. PC-basedworkstations, notebook and pen-based computers, printers and other WLANnetworkable devices can move freely in the cell. FIG. 1 depicts a basicWLAN cell 10. Each wireless LAN cell requires some communications andtraffic management. The communication and traffic management isgenerally coordinated by an access point (AP) 12, which communicateswith each wireless station 14 in its coverage area. A station 14 mayalso communicate with another station 14 via the AP 12 or without the APwhen one station is acting as an AP. The AP 12 may also function as abridge between the wireless stations 14 and a wired network and otherwireless cells (not shown). Connecting an AP 12 to a backbone or toother wireless cells can be accomplished by wire or by a separatewireless link, using wireless bridges. The range of the system ornetwork can be extended by cascading several wireless links, one afteranother.

The IEEE 802.11 protocol, which is used for WLAN communications allowsfor dynamic rate switching. The data rate of each station 14 is adjustedaccording to the channel condition. Performance (i.e., throughput ofdata) is maximized by increasing the data rate, but at the same timekeeping re-transmissions of data to a minimum. This is very importantfor WLAN mobile applications where the signal quality fluctuatesrapidly.

Embodiments of the invention require little or no modification to theIEEE 802.11 standard and are relatively simple to incorporate into adevice. Embodiments of the present rate adaptation mechanism areextremely dynamic by supporting the higher transmission rates in goodchannel conditions and by working well in all types of channelconditions without becoming unstable. The embodiments are also scaleablefor future IEEE 802.11 upgrades.

Rate adaptation mechanisms in accordance with embodiments of theinvention focus on adaptation algorithms that do not need feedbackinformation or a central control station that is not already part of theIEEE 802.11 standard. If a sending station does not receive an ACK backin response to a data frame sent to a certain receiving station, thesending station will assume that the channel condition is deterioratingor there is packet collision. On the other hand, if the sending stationsucceeds in receiving several ACKs in a row from the receiving station,the sending station might be able to assume that the channel conditionis improving and a higher data rate (data transmission rate mode) may befeasible.

Analysis of previously proposed ARF algorithms revealed that theirperformance is somewhat dependent on two key parameters: (1) being thechoice of the threshold value, i.e., after how many consecutivelyreceived ACKs should the sending station increase the data transmissionrate mode; and, after how many missed ACKs should the data transmissionrate mode be reduced. And (2) being how to search for the optimum datatransmission rate and how to know when the optimum data transmissionrate is reached.

In previous ARF rate adaptation schemes, there appear to be twoimportant values that affect their performance: the Failure Threshold(FT) and the Success Threshold (ST). The FT value stands for the numberof unreceived ACKs in a row before the transmission rate is decreased bythe sending station. The ST value stands for the number of consecutivelyreceived ACKs by the sending station before the transmission rate isincreased.

Another important value is the way and by how much the transmitting rateis increased or decreased when the ST or FT value is reached.

Using a single fixed value for the ST is very, perhaps overly, sensitiveto the rate that a channel's condition changes. A rate adaptationalgorithm with two fixed ST values, one value for when the currentchannel condition is in a higher Doppler Spread value and the other STvalue used when the channel condition in a lower Doppler spread value.It was noted that an ARF algorithm using two fixed values of STperformed better than one with a single value for ST, but a two value STadaptive algorithm is still not very adaptive to differing channelconditions. For example, if the channel condition is stable and thecurrent data transmission rate is already the best successful rate, thisrate prior art adaptive algorithm will periodically and after a fixednumber of successful data transmissions attempt to increase the datatransmission rate to the next higher rate. Increasing the datatransmission rate will cause a data transmission error such that thesending station will not receive an ACK back from the receiving station.Then, the channel rate will be decreased back to the best successfulrate. This channel transmission rate fluctuation decreases the datathroughput on the channel. It would be better to raise the ST value whena best successful rate is found in order to minimize undesired rateincrease attempts.

Furthermore, a value of FT that is too high also degrades the overalldata throughput of a channel. Various values for FT were simulated andit was found that if FT should be fixed, the best value for FT is 1.Some previous ARF adaptive rate algorithms the FT value was set to, forexample, 4 and then the data transmission rate was simply decreased by 1after the FT value was reached. Conversely, in embodiments of thepresent rate adaptive algorithm, a binary search method is used tobetter adapt the ST and FT values to channel conditions.

Through additional simulation and testing, it was observed that there isa correspondence between the nature of changing channel conditions inWLANs and the nature of changing congestion conditions on the Internet.This observation was new and unexpected because it deviates greatly fromhow previous WLAN rate adaptation algorithms operate. It was discernedthat an ARF rate adaptation scheme, in accordance with embodiments ofthe present invention, (through the proper setting of ST and FTthresholds) should be both dynamic and stable much like TCP congestioncontrol algorithms handle congestion in Internet communications. To dateno one has thought to utilize anything like a TCP algorithm for a WLANenvironment.

As such, embodiments of the present adaptive rate algorithm uses a verymodified form of Additive Increase-Multiplicative Decrease (AIMD). AIMDis used in TCP functions on the Internet to get window sizes for TCPcongestion control in Internet communications. In embodiments of thepresent rate adaptation algorithm the threshold settings ST and FT arechanged and made adaptive to WLAN channel conditions using a type ofAIMD mechanism. Other embodiments of the present invention may use aMultiplicative Increase-Multiplicative Decrease (MIMD) algorithm toadjust threshold settings. Other embodiments of the invention can use acombination of an AIMD and MIMD algorithm to set adjust the thresholdconditions.

In an exemplary embodiment of the present rate adaptation mechanismthreshold values, like ST, are dynamically updated as follows. If a WLANchannel transmission success immediately follows a transmission datarate mode increase, then the ST value is increased in order to avoidtransmission failure caused by increasing the transmission rate to ahigher mode in rapid succession. Further, if a transmission failureoccurs immediately after a transmission rate mode increase, then the STvalue is increased in order to discourage another transmission rate modeincreasing attempt.

Embodiments of the present adaptive rate mechanism start with an initialthreshold value TH. When the condition is satisfied for increasing thethreshold, then TH is increased by adding a predetermined number α.Similarly, when the condition is satisfied for decreasing the threshold,then TH is divided by a predetermined number β each time.

In summary, an exemplary rate adaptation scheme for a WLAN device adaptsthe threshold TH. TH is initially set at a predetermined number.

1. When transmitting successfully at a given data transmission rate, andthe TH condition of successful ACKs has been reached, then TH isincremented by an amount α (TH=TH+α). Through experimentation andsimulation, shown below, a preferred α is equal to 16, but can be in therange of 1 to 30.

When transmitting successfully at a given data transmission rate and noACK is received by the sending station, then TH is decremented bydividing TH by β (TH=TH/β) through experimentation and simulation, shownbelow, a preferred β is equal to 2, but can be in the range of 1 to 10.

Table 1 depicts the operating ST values of an existing IBM WLAN ARFadaptive rate algorithm (the IBM algorithm) compared with the TH valuesof an exemplary algorithm in accordance with the present invention:

TABLE 1 Data Transmission Rate 6 9 12 18 24 36 48 54 Mbps Mbps Mbps MbpsMbps Mbps Mbps Mbps IBM 10 10 9.51 9.88 9.10 9.79 8.65 N/A Present 712.48 26.3 39.1 41.9 55.5 65.5 N/A Exemplary AlgorithmNote that the TH values for the present exemplary algorithm are dynamicand track the respective channels' condition as much as possible. On theother hand, the IBM algorithm (and others) have their ST thresholds setirrespective of channel conditions and data transmission rate.

When transmission is successful enough in a channel such that the THthreshold is reached (the requisite number of consecutive ACKs have beenreceived by the sending station, the appropriate data transmission modemust be determined. The simplest thing to do is to increase thetransmission data rate mode by 1. In embodiments of the presentinvention, a binary search method is used. The binary search method isused when a threshold TH has been reached successfully (i.e., when theTH number of consecutive ACKs has been achieved). When there is afailure threshold, and it has been reached, then the data transmissionrate is decreased by 1. The rational is to locate the optimal datatransmission rate as soon as possible.

When performing the binary search, and a successful threshold isreached, the data transmission rate mode is set to a data transmissionrate mode in the middle of the previous data transmission rate mode andthe highest possible data transmission rate mode (which is presently 8).For example, if the previous data transmission rate mode is 4, then thenew data transmission rate mode will be (4+8)÷2=6. If the nexttransmission is a failure (i.e., no ACK) then the data transmission ratemode is decreased by 1. Following the example, the new data transmissionrate mode is 6−1=5. When the first transmission in a new datatransmission rate mode is successful, then the binary searching phasestops and a transmission phase begins.

The exemplary rate adaptation algorithm of the present invention can bedivided into two phases; the searching phase and the transmission phase.The exemplary algorithm starts with the searching phase and then shiftsto the transmission phase when the first transmission in a newtransmission mode is successful (i.e., an ACK is received by the sendingstation). The exemplary algorithm will stay in the transmission phaseuntil a failure (i.e., No ACK) occurs or a success threshold TH isreached.

Referring now to FIG. 2, FIG. 2 is a flow chart depicting how anexemplary rate adaptation algorithm for a WLAN operates. Since anexemplary rate adaptation algorithm has two types of transmission,transmission success and transmission failure, FIG. 2 depictstransmission success. At step 100 a transmission success on a WLANcommunication just occurred. In other words, an ACK was received by thesending station indicating that the receiving station received thetransmitted data or data frame. At step 102 it is determined whether,just prior to the successful transmission of step 100, a successfulthreshold TH had been reached and the data transmission rate mode hadjust been increased. If the answer is yes then at step. 104 the successthreshold TH is increased by a (i.e., TH−TH+α). If the answer is no,then it must be determined whether the algorithm is in the searchingphase at step 106. If the rate adaptation algorithm is in the searchingphase, it should keep or stay at the present data transmission rate modeand switch to the transmission phase (step 108). If at step 106 theexemplary rate adaptation is not in the searching phase, but instead isalready in the transmission phase, then at step 110 it is determinedwhether the number of consecutive successful transmissions (i.e.,successful ACKs) is greater than the present value of the threshold TH.If the number of successful and consecutive ACKs is less than TH, then,at step 112, the current data transmission mode is kept and thetransmission phase continues. On the other hand, if at step 110, thenumber of consecutively successful transmissions is greater than thethreshold value TH, then, at step 114, a new data transmission rate modeis selected. The new data transmission rate mode is equal to (thecurrent mode number+((the total number of available modes)−(current modenumber)÷2)). Since the present total number of available datatransmission rate modes is presently 8, then when the calculation isdone, the resulting answer may not be an integer (i.e., whole number).Thus, embodiments of the present invention may round-up, round-down, usethe integer value or round to the higher whole number in order tocalculate the new mode. For example, if the total number of availabletransmission modes is 8 and the present transmission mode is 3 and theTH has met the new mode calculation of step 14 which is newmode=3+((8−3)÷2)=5.5, thus, depending on the embodiment of the presentinvention, the new data transmission mode can be mode 5 or 6.

Now referring to a transmission failure event (i.e., No ACK received) welook at FIG. 3. At step 200 a transmission failure occurs in the channeland an ACK is not received by the transmitting station. At step 202 itis determined whether the transmission just prior to this transmissionfailure, was the threshold TH reached and the data transmission ratejust been increased to a new data transmission rate mode, then a newsuccess threshold is set as TH=TH+α. If in the transmission just priorto this transmission failure, the TH threshold was not reached, then atstep 206 it is determined whether the exemplary algorithm is insearching phase. If the exemplary algorithm is in searching mode and atransmission failure occurred, then, at step 208 a new data transmissionrate mode is set to the current mode minus 1 and the algorithm stays inthe searching phase for the next transmission at step 209.

If at step 206, the exemplary algorithm is not in the searching phase,then at step 210 the data transmission rate mode is set to a new datatransmission rate mode equal to the current data transmission rate modeminus 1 and start a new searching phase. The algorithm goes to the nexttransmission at step 209 to find out whether it will be a success orfailure. Furthermore, if at step 210, the current data transmission moderate is less than then the threshold TH is reset to TH=TH/β and theexemplary algorithm goes into a new search phase.

In other embodiments of the invention, the success threshold TH isadjusted by either an AIMD or MIMD algorithm. Other embodiments set amaximum value for the success threshold TH. For example, a maximum valuefor a success threshold TH might be a number between 20 and 200. Assuch, regardless of the success rate of communication, the TH cannot goabove fixed present maximum value. Furthermore, a minimum value of THcan be set. For example, regardless of the failure rate, the value forTH may not be able to go below some fixed minimum value. A range for theminimum TH value may be between 1 and 10.

The exemplary adaptive data rate scheme adapts its thresholds to WLANchannel conditions using a dynamic technique that is somewhat similar tothe TCP AIMD scheme, but has never been used with WLAN systems.Furthermore, compared with previous ARF or adaptive rate adjustmentschemes, embodiments of the present invention achieve better throughputperformance (shown below) than any static threshold or near staticthreshold ARF WLAN rate adjustment schemes.

Exemplary adaptive rate schemes for WLAN are more complicated than theInternet's TCP techniques because in the exemplary rate adaptive schemesboth the threshold number TH and the data transmission rate mode must bedetermined, rather than just the threshold. In the Internet's TCP, thethreshold (window) determines the rate.

Embodiments of the present invention are easily implemented within thecurrent IEEE 802.11 standard. The transmitter (i.e., sending station 14)needs to maintain for a certain MAC address two counters. One counterfor counting successful transmissions (i.e., ACK received) and a secondcounter for counting failed transmissions (i.e., no ACK received). If aframe of data is successfully transmitted, the success counter isincremented by one and the failure counter is reset to zero. Similarly,if a frame of data is unsuccessfully transmitted, the failure counter isincremented by one and the success counter is reset to zero. If thefailure counter reaches a predetermined threshold value FT, then thedata transmission rate for the corresponding destination is decreasedand the failure counter is reset to zero. Similarly, if the successcounter reaches a predetermined threshold value TH, then the datatransmission rate is increased and the success counter is reset to zero.

Embodiments of the present invention are not limited to usage on a WLANenvironment, but an exemplary rate adaptation algorithm can be appliedto any device with a wireless communication ability.

The performance of exemplary rate adaptation algorithms wereinvestigated using discrete-event simulations. The simulator was builtusing MATLAB with Simulink, communications toolbox, communicationsblockset, DSP blockset and signal processing tool box software. Thesimulator models the 802.11a PHY and MAC layers.

Stationary nodes were considered in the simulation. A fixed transientpower level was assumed along with the assumption that the link SNR(signal-to-noise ratio) can be specified on the fly. The Jakes'simulation model was used to implement the multipath Rayleigh FadingChannel. The simulation allowed for the maximum Doppler shift to bespecified before running an algorithm.

An end-to-end physical layer and 4 modulation schemes with 8 datatransmission rate modes were implemented. The 8 data transmission ratemodes were 6, 9, 12, 18, 24, 36, 48, and 54 Mb/s. In the simulation, itwas assumed that random data transmissions occurred without datascrambling. The bit err rate (BER) was obtained by simulation.

Table 2 shows the parameters used in an exemplary algorithm of asimulation.

TABLE 2 Parameters used in the PHY layer simulation OFDM symbols perTransmission Block 20 Number of OFDM symbols in training sequence  4Viterbi traceback depth 34 Symbol period (second) 0.08e−6 InitialSuccess Threshold 16 Failure Threshold  1 Addition α 16 Division β  2

Table 3 shows the parameters used in the simulation of the MAC layer

TABLE 3 Parameters used in the MAC layer simulation tSlotTime 9μ Slottime tSIFSTime 16 μs SIFS time tDIFSTime 34 μs DIFS time aCWmin 15 Mincontention window size aCWmaz 1023 Max contention window sizetPLCPPreamble 16 μs PLCP preamble duration tPLCP_SIG  4 μs PLCP SIGNALfield duration tSymbol  4 μs OFDM symbol internal

In the MAC layer, only the distributed coordination function (DCF) isconsidered. Only data packets and some control frames (RTS, CTS, ACK)necessary for the DCF are considered. In the simulation, the performanceof the exemplary rate adaptation algorithm was compared to that of theIBM algorithm, the WaveLAN algorithm and the following three ARFalgorithms:

ARF_(—)1: After 4 missed ACKS in a row, this algorithm goes intofallback state (decreases the transmission rate mode by one); then after5 consecutive ACKs recovering by (increasing the data transmission ratemode.

ARF_(—)2: After 4 missed ACKs in a row, this algorithm goes intofallback state; then after 5 consecutive ACKs are received, a probationstate is entered. If the next ACK is missed, then this algorithm goesback to the fallback state, otherwise it begins recovering.

ARF_(—)3: This test algorithm is similar to ARF_(—)2, except 11 Acks arerequired consecutively to go into recovery.

In order to cover a wide variety of WLAN networking environments,experiments were performed with the following varying parameters: 1)differing channel models: flat fading, non-flat fading, and dispersivefading; 2) different Doppler spreads: 1 Hz, 5 Hz, 10 Hz, and 20 Hz; 3)different packet collision conditions: no collision and with collision;4) different SNR values: from 8 dB to 24 dB; 5) single sender andmultiple sender; and 6) comparison of the exemplary rate adaptationalgorithm with “signal measurement”: base schemes.

The following tables and associated figures present results of thecomparative experiment in the various networking environments. Thetables provide the overall differenced performance between an exemplaryadaptive rate algorithm and other algorithm/schemes. It should be notedthat in all circumstances, the present exemplary embodiment performedbetter by providing a higher average throughput in Mbps than any of thecompeting algorithms and schemes.

FIGS. 4, 5, 6 and 7 provide a graph showing the data throughput in Mbpsv. SNR in dB for various tested algorithms, including an exemplary rateadaptation algorithm, with flat fading/Doppler spread=1 Hz, 5 Hz, 10 Hz,and 20 Hz, respectively. Tables 4, 5, 6, and 7 provide the throughput inMbps of each algorithm and the percent (%) improvement throughput of anexemplary rate adaptation algorithm over the other tested algorithms.

TABLE 4 Throughput improvement (%) of present algorithm with respect toother algorithms FLAT Fading/Doppler Spread - 1 Hz SNR IBM WaveLAN ARF_1ARF_2 ARF_3 24 Mbps 10 dB 7 16.2 67 55.5 30.6 105 12 dB 13.9 28.5 87.171.6 39.6 11 14 dB 14.2 24.5 89.1 74.5 48.3 2.6 16 dB 19.2 33.1 103.291.3 49.1 23.4 18 dB 9.3 18.2 79.1 68 34 40 20 dB 6.7 12.3 42.3 37.723.8 75.6 Average 11.72% 22.1% 77.97% 66.25% 37.52% 42.93% IncreasedThrough- put

TABLE 5 Throughput improvement (%) of present algorithm with respect toother algorithms FLAT Fading/Doppler Spread - 5 Hz SNR IBM WaveLAN ARF_1ARF_2 ARF_3 24 Mbps 14 dB 8.12 19.59 80.14 65.189 26.97 135.67 16 dB9.29 18.46 80.19 66.92 30.51 62.04 18 dB 7.64 18.17 67.48 58.44 29.6941.45 20 dB 8.61 16.31 49.82 41.58 22.83 44.10 22 dB 9.04 15.97 57.5351.57 28.16 61.02 24 dB 8.43 13.55 48.99 43.53 26.33 77.92 Average 8.53%17.34% 64.03% 54.54% 27.42% 70.37% Increased Through- put

TABLE 6 Throughput improvement (%) of present algorithm with respect toother algorithms FLAT Fading/Doppler Spread - 10 Hz SNR IBM WaveLANARF_1 ARF_2 ARF_3 24 Mbps 14 dB 9.23 18.80 70.32 60.59 26.26 239.59 16dB 10.23 21.36 86.92 74.67 30.72 73.31 18 dB 7.43 16.34 72.66 63.0425.00 14.29 20 dB 11.12 21.74 76.05 64.89 31.84 25.27 22 dB 7.96 15.6066.50 56.48 27.07 40.83 24 dB 9.23 17.08 71.03 62.59 36.02 63.33 Average9.20% 18.49% 73.91% 63.71% 29.48% 76.10% Increased Through- put

TABLE 7 Throughput improvement (%) of present algorithm with respect toother algorithms FLAT Fading/Doppler Spread - 20 Hz Wave- SNR IBM LANARF_1 ARF_2 ARF_3 24 Mbps 14 dB 7.87 18.35 111.43 96.29 42.86 179.25 16dB 8.82 19.34 108.09 90.46 40.19 93.55 18 dB 9.47 20.30 110.97 92.9141.23 42.95 20 dB 11.03 19.93 95.02 83.62 42.55 39.96 22 dB 5.66 12.9966.84 59.38 29.02 45.21 24 dB 2.63 2.63 53 46.97 23.63 60.98 Average7.58% 15.76% 91.04% 78.27% 36.58% 76.98% Increased Through- put

FIG. 8 provides a graph showing the data throughput in Mbps v. SNR in dBfor various tested algorithms, including an exemplary rate adaptationalgorithm, with no fading channel (Doppler spread=1, 5, 10 and 20 HZ).Table 8 provides the throughput in Mbps of each algorithm tested and thepercent (%) improvement throughput of an exemplary rate adaptationalgorithm over the other algorithms.

TABLE 8 Throughput improvement (%) of present algorithm with respect toother algorithms Flat Fading/Doppler Spread - 1 Hz, 5 Hz, 10 Hz, and 20Hz SNR IBM WaveLAN ARF_1 ARF_2 ARF_3 24 Mbps 10 dB 16.94 36.25 166.13146.27 70.63 1962.50 12 dB 9.68 15.01 110.11 90.82 31.41 −10.82 14 dB22.27 29.48 193.02 162.57 83.88 −2.00 16 dB 18.21 17.39 94.24 82.2128.21 12.50 18 dB 19.57 37.05 189.32 158.26 77.87 43.33 20 dB 11.5213.13 107.91 94.61 36.30 53.33 Average 16.36% 24.72% 143.46% 122.28%54.72% 343.14% Increased Throughput

FIGS. 9, 10 and 11 each provide a graph showing the data throughput inMbps v. SNR in dB for various tested algorithms, including an exemplaryembodiment of the present invention (our algorithm), with dispersivefading/Doppler spread=1 Hz, 5 Hz, and 10 Hz, respectively. Tables 9, 10and 11, below, provide the throughput in Mbps of each algorithm testedand the percent (%) improvement throughput of the exemplary embodimentwith respect to the other tested algorithms having dispersive fading andDoppler spreads of 1, 5, and 10 Hz, respectively.

TABLE 9 Throughput improvement (%) of present algorithm with respect toother algorithms DISPERSIVE Fading/Doppler Spread - 1 Hz SNR IBM WaveLANARF_1 ARF_2 ARF_3 24 Mbps  8 dB 12.5 22.73 92.86 80 42.11 N/A 12 dB11.28 26.28 117.65 104.42 59.83 N/A 16 dB 8.75 16.35 103.17 85.51 40.66152.96 20 dB 15.88 26.28 116 105.21 55.12 10.06 24 dB 13.97 23.08 121.54102.82 52.38 20.50 Average 12.48% 22.95% 110.25% 95.59% 50.12% 61.17%Increased Throughput

TABLE 10 Throughput improvement (%) of present algorithm with respect toother algorithms DISPERSIVE Fading/Doppler Spread - 5 Hz Wave- 24 SNRIBM LAN ARF_1 ARF_2 ARF_3 Mbps  8 dB 13.85 24.06 100.76 85.21 43.72 N/A12 dB 14.07 36.05 185.71 104.48 70.45 N/A 16 dB 12.19 21.59 100.69 86.8046.84 51.83 20 dB 11.75 25.90 117.50 102.80 60 22.54 24 dB 4.92 9.1379.16 69.62 26.46 21.94 Average 11.36% 23.35% 116.77% 89.58% 49.51%31.1% Increased Through- put

TABLE 11 Throughput improvement (%) of present algorithm with respect toother algorithms DISPERSIVE Fading/Doppler Spread - 10 Hz Wave- SNR IBMLAN ARF_1 ARF_2 ARF_3 24 Mbps  8 dB 5.26 12.99 70.94 60 27.39 N/A 12 dB11.75 23.14 85.48 50 43.73 N/A 16 dB 10.18 17.35 87.76 67.27 31.81 66.6720 dB 7.96 21.45 105.43 91.48 49.33 41.54 24 dB 8.89 15.01 89.30 79.8436.39 57.04 Average 8.81% 17.99% 87.78% 69.72% 37.73% 55.08% IncreasedThrough- put

In other experiments it was determined that embodiments of the presentinvention provide a 9.15% throughput improvement over the IBM algorithmin conditions of flat fading with collisions and a Doppler spread of 5Hz. Embodiments of the present invention, in yet other experiments,showed an average 14.68% throughput improvement over the IBM algorithmwhen there existed flat fading with multiple nodes and a Doppler spreadof 5 Hz.

Although preferred embodiments of the method and apparatus of thepresent invention have been illustrated in the accompanying Drawings anddescribed in the foregoing Detailed Description, it will be understoodthat the invention is not limited to the embodiments disclosed, but iscapable of numerous rearrangements, modifications and substitutionswithout departing from the spirit of the invention as set forth anddefined by the following claims.

1. A Wireless LAN system (WLAN) comprising: a sending station programmedto send WLAN communications; a receiving station programmed to receiveWLAN communications from, at least said sending station; said sendingstation programmed to send said WLAN communications using an adaptiverate algorithm, said adaptive rate algorithm having a searching mode forsearching for a data transmission rate mode that provides successfulWLAN communication between said sending station and said receivingstation; said adaptive rate algorithm having a transmission mode whereinsaid data transmission rate mode remains the same until a predeterminednumber of unsuccessful communication attempts occur, wherein when insaid transmission mode, said data transmission rate mode is increased toa new data transmission rate mode equal to ((a current data transmissionrate mode number)+((a total number of possible data transmission ratemodes)−(the current data transmission rate mode number)) / 2) when athreshold number (TH) of successful data frame transmissions areconcurrently made.
 2. The WLAN system of claim 1, wherein said searchingmode comprises a success threshold number (TH) that is increased by apredetermined amount (α) when TH concurrent, successful transmissionsare made by said sending station.
 3. The WLAN system of claim 1, whereinsaid searching mode comprises the threshold number (TH) that isdecreased by a predetermined amount TH/β when a transmission failureoccurs between said sending station and said receiving station, whereinβ is a selected number.
 4. The WLAN system of claim 1, wherein when insaid transmission mode, said data transmission rate mode is decreased byone when a predetermined number of unsuccessful data frame transmissionsare made.
 5. The WLAN system of claim 4, wherein said predeterminednumber of unsuccessful data frame transmission is one.
 6. The WLANsystem of claim 1, wherein said new data transmission rate mode is aninteger, rounded up to the nearest whole number, rounded down to thenearest whole number, or rounded off to a whole number in order todesignate the new data transmission rate mode.
 7. The WLAN system ofclaim 2, wherein α is a number equal to or between 1 and
 30. 8. The WLANsystem of claim 3, wherein β is a number between 1 and
 10. 9. The WLANsystem of claim 1, wherein said WLAN system uses an IEEE 802.11standard.
 10. A method of communicating in a WLAN system comprising:sending a data frame, by a sending device, to a receiving device, saidsending is performed in a current data transmission rate mode;determining whether an acknowledge signal is received at said sendingdevice; if said acknowledge signal is received at said sending device,incrementing a success threshold counter by one; if said successthreshold counter is equal to a success threshold number (TH), thenincreasing said current data transmission rate mode to a new datatransmission rate mode equal to ((a current data transmission rate modenumber)+((a total number of possible data transmission rate modes)−(thecurrent data transmission rate mode number))/2) and resetting saidsuccess threshold counter to zero; if said success threshold counter isnot equal to said success threshold number, then incrementing saidsuccess threshold counter by one and sending a next data frame from saidsending device to said receiving device.
 11. The method of communicatingin a WLAN system of claim 10, further comprising: determining that anacknowledge signal was not received at said sending device; if saidacknowledge signal is not received at said sending device, and ifimmediately prior to the sending step said data transmission rate modewas increased, then increasing said success threshold number (TH) by apredetermined amount α.
 12. The method of communication in a WLAINsystem of claim 11, wherein a is a number from 1 to
 30. 13. The methodof communication in a WLAIN system of claim 11, further comprising:determining whether said method is in a searching phase or transmissionphase; if said method is in said searching phase, then setting saidcurrent data transmission mode to said current data transmission mode−1if the current data transmission rate mode in this transmission phase isless than the previous current transmission mode in the previous datatransmission rate mode, then resetting the success threshold number(TH)−TH/β and resetting said success threshold counter to zero.
 14. Themethod of communication in a WLAN system of claim 11, furthercomprising; determining whether said method is in a searching ortransmission phase; if said method is in said transmission phase, thensetting a new current data transmission rate mode=the current datatransmission rate mode−1; if the current data transmission rate mode inthis transmission phase is less than the previous current transmissionmode in the previous data transmission rate mode, then resetting thesuccess threshold number (TH)=TH/β and resetting said success thresholdcounter to zero, wherein β is a selected number.
 15. The method ofcommunication in a WLAN system of claim 14, wherein β is number between1 and
 10. 16. The method of communication in a WLAN system of claim 10,wherein said WLAN system uses the IEEE 802.11 standard.
 17. A WLANsystem comprising: a sending device for sending wireless communicationsin a WLAN network on a channel using an adaptive rate algorithm having asearching mode and a transmission mode; wherein in said transmissionmode, a data transmission rate mode is increased to a new datatransmission rate mode equal to ((a current data transmission rate modenumber)+((a total number of possible data transmission rate modes)+(thecurrent data transmission rate mode number))/2) when a threshold number(TH) of successful data frame transmissions are concurrently made. 18.The WLAN system of claim 17, wherein said algorithm adaptivelydetermines a number of consecutive successful data frame transmissionthat must occur prior to increasing the data transmission rate mode ofsaid sending device on said channel.